Excavation is often done to expose an underground object, such as a pipe, for maintenance or replacement. Digging as close to the object being sought with automated digging machines, such as back hoes, maximizes the efficiency of excavation. However, if the location of the object is not determined accurately enough, damage may occur. Furthermore, there may be other objects to be avoided in the excavation process, even if the excavation isn't exposing existing infrastructure. If the objects underground are not known or adequately located, the excavation team may have to limit the use of automated digging equipment to avoid damaging the objects, and spend a significant amount of time manually digging with shovels.
One solution to this problem has been to use injected radiation. In this process, an RF signal is injected into the object at a place where it breaks the surface, and the rest of the object, typically a pipe is tracked by a device on the surface that is sensitive of the radiated signal. However, injected radiation requires a portion of the object to be exposed above the ground's surface so can be energized, and also requires the object to be conductive.
Another solution to this problem involves Ground Penetrating Radar (GPR) in which the radar signals are transmitted into the ground at a perpendicular angle. GPR is sensitive to a broader range of objects as it relies only on differences in dielectric constant between the object and the surrounding ground to generate returns, which is not limited to conductive objects. However, this technique has not always provided adequate resolution or accurate depth measurements of the objects in the ground.
An alternative GPR method utilizes an array of antenna positions from a single point of view in order to form a synthetic aperture, typically from an oblique angle. The oblique angle is used primarily to enable the array of antenna positions to be made uniform by employing an antenna structure, which would be cumbersome if done at while wheeled or towed along the ground as occurs with the standard form of GPR. Synthetic aperture improves accuracy and resolution somewhat from standard GPR, but not to the degree needed in excavation work.